Aridland riparian ecosystems are limited in size and availablility, the climate is changing, and further hydrological change is likely in the American Southwest. To protect riparian ecosystems and organisms, scientists and land managers need to understand how disturbance processes and stressors such as fire, drought, and non-native plant invasions affect them. Riparian vegetation is critically important as foraging, resting, migrating, and breeding habitat to birds and other animal species in the southwestern United States. To address these issues, USFS Rocky Mountain Research Station Grasslands, Shrublands, and Desert Ecosystems Science program manager Deborah Finch and USFS Region 3 Research Associate D. Max Smith, reviewed the ecohydrology of southwestern streams along the Middle Rio Grande to describe the effects of hydrological changes, wildfire, and invasions on plant communities and riparian-nesting birds. They examined climate change projections and output from population models to gauge the future of aridland riparian ecosystems in an increasingly arid Southwest.
The structurally diverse, species-rich vegetation along many southwestern streams supports high densities of territories and nest sites for a variety of birds including several species of high conservation priority. Survival and reproduction of woody riparian plants may depend on periodic floods and droughts. Woody species, such as Fremont cottonwood (Populus fremontii) and Arizona sycamore (Platanus wrightii), among others, provide birds with nesting sites and foraging opportunities that may be absent or rare in adjacent plant communities. In addition, as in other regions, human activity has heavily altered rivers and streams of the American Southwest, resulting in significant changes to disturbance regimes.
Finch and Smith found that their model supports the contention that Middle Rio Grande cottonwood forests are in decline and will be replaced by other woody species by the end of this century. Nonnative woody species such as Russian olive and saltcedar are present throughout the study area and will likely increase in abundance as cottonwood declines. Replacement of cottonwood by these nonnatives will change the structure of the Middle Rio Grande riparian forest by increasing the density of low-stature vegetation and decreasing canopy height. Hydrological models, incorporating greenhouse gas emission scenarios, project that these and other changes will worsen with climate change. You can read about all of their findings in the latest RMRS General Technical Report.